U.S. patent application number 10/448081 was filed with the patent office on 2003-10-30 for low signal loss bonding ply for multilayer circuit boards.
This patent application is currently assigned to TONOGA, INC.. Invention is credited to McCarthy, Thomas F., Wynants, David L. SR..
Application Number | 20030203174 10/448081 |
Document ID | / |
Family ID | 46282394 |
Filed Date | 2003-10-30 |
United States Patent
Application |
20030203174 |
Kind Code |
A1 |
McCarthy, Thomas F. ; et
al. |
October 30, 2003 |
Low signal loss bonding ply for multilayer circuit boards
Abstract
A process for fabricating a low loss multilayer printed circuit
board using a bonding ply comprising a reinforced fluoropolymer
composite and a high-flowing thermosetting adhesive composition is
disclosed. The fluoropolymer composite comprises at least one
fluoropolymer and a substrate selected from woven fabrics, nonwoven
fabrics and polymeric films.
Inventors: |
McCarthy, Thomas F.;
(Bennington, VT) ; Wynants, David L. SR.;
(Cambridge, NY) |
Correspondence
Address: |
HESLIN ROTHENBERG FARLEY & MESITI PC
5 COLUMBIA CIRCLE
ALBANY
NY
12203
US
|
Assignee: |
TONOGA, INC.
136 Coonbrook Road
Petersburgh
NY
|
Family ID: |
46282394 |
Appl. No.: |
10/448081 |
Filed: |
May 29, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10448081 |
May 29, 2003 |
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10289984 |
Nov 7, 2002 |
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10289984 |
Nov 7, 2002 |
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09952486 |
Sep 14, 2001 |
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6500529 |
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Current U.S.
Class: |
428/209 |
Current CPC
Class: |
Y10T 428/24994 20150401;
H05K 3/4626 20130101; H05K 2201/015 20130101; H05K 2201/0195
20130101; Y10T 428/25 20150115; H05K 1/036 20130101; H05K 2201/0278
20130101; H05K 1/034 20130101; H05K 2201/0209 20130101; H05K
2201/029 20130101; H05K 1/0366 20130101; Y10T 428/24917 20150115;
Y10T 428/2804 20150115; H05K 3/386 20130101 |
Class at
Publication: |
428/209 |
International
Class: |
B32B 003/00 |
Claims
In the claims:
1. A low loss circuit board material having a dissipation factor of
less than 0.005 and a dielectric constant of less than 4.0, said
low loss circuit board material comprising 0.025 to 0.2
lbs/yd.sup.2 of a high flowing thermosetting adhesive composition
disposed on a fluoropolymer composite comprising a fluoropolymer
composition comprising a ceramic dielectric modifier and disposed
on a substrate selected from nonwoven fabrics, polymeric films and
woven fabrics impregnated with a filler-free fluoropolymer
composition.
2. A low loss circuit board material according to claim 1, wherein
the fluoropolymer composition comprises a plurality of sublayers
comprising ceramic dielectric modifier distributed in the
fluoropolymer.
3. A low loss circuit board material according to claim 1, wherein
the high flowing thermosetting adhesive composition has a precure
melt viscosity of less than 100 Pa*s over a temperature range of
20.degree. C. to 200 C.
4. A low loss circuit board material according to claim 1, wherein
the high flowing thermosetting adhesive composition comprises an
epoxy resin.
5. A low loss circuit board material according to claim 4, wherein
the high flowing thermosetting adhesive composition additionally
comprises an epoxy-functional reactive diluent.
6. A low loss circuit board material according to claim 1, wherein
the fluoropolymer composition comprises PTFE.
7. A low loss circuit board material according to claim 1, wherein
the ceramic dielectric modifier comprises silica.
8. A low loss circuit board material according to claim 1, wherein
the fluoropolymer composition additionally comprises a filler other
than the ceramic dielectric modifier.
9. A low loss circuit board material according to claim 8, wherein
the filler comprises a crosslinked PTFE filler.
10. A low loss circuit board material according to claim 1, wherein
the substrate comprises a woven fabric.
11. A low loss circuit board material according to claim 1, wherein
the substrate comprises a woven fiberglass fabric.
12. A low loss circuit board material according to claim 1, wherein
the fluoropolymer composite is perforated by laser or mechanical
means before the adhesive layer is applied.
13. A low loss circuit board material according to claim 1, wherein
a surface of the fluoropolymer composite is treated to enhance
adhesion before the adhesive layer is applied.
14. A low loss laminate comprising at least one low loss circuit
board material according to claim 1.
15. A low loss laminate according to claim 14, wherein the low loss
circuit board material is bonded to a non-adhesive layer.
16. A low loss laminate according to claim 14, wherein the
non-adhesive layer is sandwiched between at least two bonding
plies.
17. A low loss laminate according to claim 14, wherein the
non-adhesive layer is selected from a fluoropolymer film and a
fluoropolymer composite comprising a fiberglass substrate coated
with a fluoropolymer.
18. A low loss laminate according to claim 14, wherein the
non-adhesive layer comprises a fiberglass substrate coated with a
fluoropolymer.
19. A low loss laminate according to claim 14, wherein the
non-adhesive layer comprises a fluoropolymer film.
20. A low loss laminate according to claim 14, wherein the
non-adhesive layer comprises a thermoset impregnated fiberglass
21. A low loss laminate according to claim 20, wherein the
thermoset comprises an epoxy, an elastomer, a polyester, an
anhydride, a bismaleimide, or a cyanate ester.
22. A low loss laminate according to claim 14, where the
non-adhesive layer additionally comprises a reinforcement.
23. A low loss laminate according to claim 14, additionally
comprising at least one metallization layer.
24. A process for fabricating a low loss multilayer printed circuit
board comprising: providing at least one low loss circuit board
material according to claim 1; and laminating together a plurality
of printed circuit board layers by means of said at least one low
loss circuit board material.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of copending U.S.
application, Ser. No. 10/289,984, filed Nov. 7, 2002, which is a
division of U.S. application Ser. No. 09/952,486, filed on Sep. 14,
2001, now issued as U.S. Pat. No. 6,500,529, the priorities of
which are claimed herein to the extent allowed. The entire
disclosures of both applications are incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] This invention relates to a multilayer bonding prepreg
comprising a fluoropolymer, a reinforcement typically consisting of
fiberglass to reinforce the low signal loss fluoropolymer, a
surface coated thermosetting resin and a ceramic dielectric
modifier to control the coefficient of thermal expansion and reduce
the resulting dissipation factor. This invention further relates to
a bonding ply composition that has the ability to reform during
lamination such that the PTFE-fiberglass layer has the ability to
conform to the outline of copper circuitry, thus reducing the mass
of thermosetting material required to fill circuitry. It has been
unexpectedly found that one can influence the ability of the
PTFE-fiberglass layer to conform around copper circuitry by
designing a thermosetting adhesive composition of very low
viscosity. Ceramic is used in either the fluoropolymer coated glass
component, the thermosetting resin surface component, or in both
components. The composite is used as a low signal loss bonding ply,
Df<0.005, Dk<4.0, that can be pressed at low temperatures to
manufacture a multilayer circuit board for high frequency
applications. The invention also relates to a copper clad laminate
that may comprise the following, a low loss prepreg
(fluoropolymer-reinforcement-thermosetting resin), another low loss
substrate (fluoropolymer-reinforcement), a reinforcement layer that
may or may not contain a thermosetting resin, a film that may or
may not contain fluorine.
BACKGROUND OF THE INVENTION
[0003] in the electronics industry multilayer circuit boards are
prepared by bonding a layer of incompletely cured thermosetting
resin reinforced with fiberglass between layers of a fully cured
print and etched laminate. For a multilayer epoxy based circuit
board, first an epoxy coated fiberglass composite is laminated with
thin copper foil on both sides. The copper is patterned using
conventional printed circuit board manufacturing processes. This
layer is referred to as the inner layer. These innerlayer print and
etched copper laminates are then bonded together typically using an
FR-4 prepreg (a flame retarded partially cured sheet of epoxy
coated fiberglass that has no copper foil cladding) using the
partially cured epoxy as an adhesive layer by pressing the
construction together in a press at temperatures such as
360.degree. F. (182.degree. C.) for two hours at 200 psi, thereby
fully curing the epoxy FR-4 adhesive layer. A composite is thereby
created having non-pattered copper layers at the surfaces and
patterned inner layers being separated by the adhesive layer. The
top and bottom non-patterned copper layers (the outer layers) can
then be print and etched yielding a multilayer circuit board.
[0004] One drawback of using many conventional thermosetting resins
as the adhesive layer is the poor electrical properties of the
bonding adhesive layer. Epoxy based thermosetting resin, for
example, has poor electrical loss characteristics in the 1-100
gigahertz range. For very long trace lengths, signal degradation
forces the use of lower loss dielectrics. This is increasingly
becoming the case for high speed digital applications (routers,
backplanes, motherboards and daughter boards). For the RF and mm
wave frequencies, polytetrafluoroethylene (PTFE) based materials
are traditionally used to prevent signal loss. PTFE based materials
have been available for a long time for the most demanding low
signal loss applications but have been avoided for cost
considerations. Conventional thermosetting resins have too high a
loss tangent at the high frequencies and are nearing their ultimate
limits at 2.5 GHz. As frequencies extend to the 5 and 10 GHz range,
it is likely that epoxy resins will be replaced by higher
performing materials. In the last 10 years epoxies were acceptable
up to 2-3 GHz but seem to be being designed out as designs move to
5 GHz. Suppliers of epoxy laminate have been reducing the loss
tangent of their products by switching to lower loss polyphenylene
oxide based polymers and ceramic fillers. Typical fiberglass based
PTFE products have 0.001-0.004 loss tangents, depending on
fiberglass wt%, versus 0.007-0.014 for modified epoxies and related
materials (10 GHz). As signal integrity drives the use of higher
performing materials, epoxy based solutions will eventually fall
short even with high loadings of ceramics or the addition of lower
loss modifying resins.
[0005] The dielectric constant is less critical but it is desired
to be below 4.0. At the high frequencies the market is largely
driven by dissipation factor and the dielectric constant is taken
for granted. Backpanels and daughter cards have been growing in the
number of layers due to the need to eliminate crosstalk between
circuitry. Lower dielectric constants lead to thinner dielectric
spacings. By designing using lower and lower dielectric constant,
the engineer can increase the number of layers yet not increase the
total overall pwb thickness if the dielectric thickness of the
individual layers can be reduced by using lower dielectric constant
materials.
[0006] An alternative solution is the use of expanded PTFE that has
been filled with epoxy and ceramic, thereby diluting the
concentration of the higher loss epoxy component. This combination
of epoxy, ceramic, and PTFE results in a sufficiently low loss
product to be acceptable for high speed digital applications. The
downside is that the expanded PTFE based solution is quite
expensive and there are issues of dimensional movement that becomes
significant with increasing layer count. U.S. Pat. Nos. 4,985,296;
4,996,097; 5,538,756; and 5,512,360 awarded to W. L. Gore describe
the use of a thermosetting resin impregnated into an expanded PTFE
web. These patents teach the use of incorporating ceramic in the
PTFE expanded web manufacture and/or part of the non-fluorinated
adhesive resin system to obtain low loss materials.
[0007] Ceramic filled resin systems based on polybutadiene-woven
fiberglass based prepregs, both filled and unfilled with flame
retardant additives, are known to be relatively low loss materials
(U.S. Pat. No. 5,571,609). These materials suffer from the
inconsistent quality of the peroxy cured rubber system and the poor
bond strengths of the cured rubber to copper foil. A related
material, crosslinked polyesters filled with kaolin, has attractive
dielectric properties but unattractive peel strengths and other
fabrication problems.
[0008] Polyphenylene oxide (PPO, APPE, PPE) based resin systems
that are cured systems of low molecular weight PPO and epoxy resins
have some process limitations (U.S. Pat. No. 5,043,367; 5,001,010;
5,162,450) for high-speed digital or high frequency applications.
Their loss tangents in the gigahertz frequency range are reported
to be in the 0.006-0.008 range. This is an improvement over
standard epoxy but their lack of flow has led to their withdrawal
from the marketplace.
[0009] Very low loss solutions include PTFE based materials and
optical interconnects. Solutions containing pure PTFE based
adhesive layers have the disadvantage that these materials need to
be processed at temperatures exceeding 700.degree. F. (fusion
bonding, 371.degree. C.). There are fabricators today building
multilayer structures based on fluorinated resin systems. Most
fabricators do not have equipment capable of pressing at these
temperatures, nor are the extended heating and cooling attractive
to fabricators. High temperature pressing on a 34-layer count stack
up could result in decreased reliability of plated through holes,
PCB warping, and copper pad distortion. In high speed digital
applications, via holes and stubs are a real source of signal loss.
The number one obstacle for high speed digital applications is the
high layer count stack-up that encourages OEMs to source board
materials that are process friendly. For high speed digital
applications, the high frequency materials may be separated from
the standard FR-4 lower frequency layers. This may lead to multiple
lamination cycles. Fabricators prefer to press laminates relatively
quickly at conventional epoxy pressing temperatures below
350.degree. F. (177.degree. C.) and have scaled their pressing
capacity so that it is not a bottleneck in the entire printed
circuit board fabrication process. Thus FR-4 is a material of
choice. However, increasing operating frequencies demand materials
having lower loss characteristics. Therefore, a composite that
enables multilayer lamination at epoxy processing temperatures that
has a minimum component of a hydrocarbon resin is especially
desirable.
[0010] One approach is to combine a PTFE-fiberglass composite with
a traditional FR-4 epoxy impregnated fiberglass within a laminate
or between two layers of copper. This has been described in
WO0011747. The advantage of this approach is that it is not
necessary to treat the surface of the PTFE-fiberglass with a layer
of thermosetting adhesive. The disadvantage of this approach is
that the thermosetting resin is combined with a fairly lossy
fiberglass reinforcement. Because adhesion to the copper is
required at the processing temperatures of conventional
thermosetting resins, an FR-4 layer would be required between the
copper layer and the PTFE-fiberglass layer. The simplest embodiment
would comprise a copper layer, FR-4 layer for bonding,
PTFE-fiberglass, FR-4 layer, copper. This approach would be
somewhat challenged to obtain a thin core laminate reaching very
low dissipation factors and 3-5 mil dielectric spacings.
[0011] Disclosed in this invention is a fluoropolymer coated
fiberglass composite that is used as the component to deliver low
signal loss properties. The fluoropolymer coated fiberglass
composite is then surface treated to enable it to bond to other
surfaces. Surface treatment is conducted on the nanometer scale in
order to maintain the desirable bulk properties of the
fluoropolymer. The surfaces of the chemically modified sheet of
fluoropolymer-coated glass are then coated with a thin layer of a
thermosetting resin which may or may not contain a ceramic
dielectric modifier or other filler (refer to FIG. 1). Although the
thermosetting resin may represent a compromise to the otherwise
good electric properties of the PTFE coated fiberglass, the
thermoset enables the manufacture of a multilayer laminate at
conventional epoxy processing temperatures. The thermosetting resin
is cured to a minimum extent possible (B-staged) during the coating
of the thermoset onto a fluoropolymer composite comprising a
substrate selected from woven fabric, non-woven or a polymeric
film. It is preferred that the thermosetting layer is dried onto
the low loss substrate with the ideal condition being no degree of
cure. This is limited in a real life manufacturing sense because
the solvent must be driven from the prepreg in an economically
viable fashion that requires forced hot air to drive the solvent
off to a lower limit that is acceptable by IPC standards. The
electrical properties of the resulting prepreg are then determined
by the ratio of the coated thermosetting resin to the
fluoropolymer-coated fiberglass starting material. It is preferred
to limit the amount of thermosetting resin to just enough to fill
the spaces between the copper traces of the inner layers and still
obtain a good bond. When the low loss substrate comprises PTFE,
fiberglass, and/or a ceramic additive, no flame retardant is
required ensuring a low dissipation factor at high frequencies.
[0012] One significant challenge is to obtain a low dielectric
constant, low dissipation factor, low cost, and good thermal
reliability. Japanese unexamined patent application 60-235844
teaches a composition comprising a PTFE woven glass substrate.
However, no regard for the dissipation factor of the composition is
given. 10 years ago most thermosetting resins would satisfy the
dissipation factor for the low frequencies in use. Combining a
PTFE-fiberglass substrate with a thermosetting resin requires a
special attention to the gap filling ability of the prepreg to fill
gaps between print and etched circuitry, the flow rheology of the
thermoset to fill those spaces, the loss factor (Df) desired by the
designers, and therefore the balance between the ratio of the
PTFE-fiberglass layer, the low loss substrate layer, to the
thermosetting layer. A given amount of thermosetting resin is
necessary to fill one and 2 oz circuitry. At least a mil of
thermosetting resin is necessary to fill 1 oz circuitry and about
2.0 mil is necessary to fill 2 oz circuitry. For a composition
where a thermosetting resin is deposited onto the two faces of a
low loss substrate that has no porosity, only thermosetting resin
that is deposited on one face of the substrate is capable of
filling gaps in copper circuitry on that respective side. In other
words, it cannot be anticipated that thermosetting resin would flow
through a non porous low loss substrate to fill gaps in copper
circuitry on the opposite side of the low loss substrate. Because a
flame retardant thermosetting layer can be anticipated to be the
highest loss component, the careful choice of a low dielectric
constant ceramic modifier will help reduce the dissipation
factor.
[0013] High speed digital applications demand thin substrates with
dielectric constants less than 4.0, dissipation factors closing in
on 0.005, dielectric spacings of 3 to 4 mil, and thermally reliable
substrates. FR-4, for example has a dissipation factor of 0.018 at
10 GHz. A construction of 1 mil of FR-4, for example, disposed on
each side of a chemically modified 2 mil PTFE-fiberglass substrate,
would be expected to have a dissipation factor in excess of 0.01.
For such a composition to reach a dissipation factor of 0.005 or
less, the chemical structure of the thermosetting layer must be
carefully chosen, and the balance between the very low loss
PTFE-fiberglass layer and the higher loss thermosetting layer, must
be carefully controlled. Prior art must be carefully evaluated as
to whether it meets the requirements of a dissipation factor less
than or about 0.005, and prescribe what is required. For a given
PTFE-fiberglass laminate, the dissipation factor of the laminate
will be dependent on the concentration of the high loss fiberglass.
However, if the PTFE concentration is kept constant and the high
loss fiberglass is replaced with a low loss ceramic material of
suitable dielectric constant, a reduction in dissipation factor can
be achieved. WO0011747 and other prior art do not suggest that the
dissipation factor of the composition can be reduced by
incorporating low loss, low dielectric constant ceramic additives
to the thermosetting layer. It also does not suggest a process of
combining a low loss substrate (PTFE-fiberglass) or plurality of
such prepregs with a prepreg comprising a fluoropolymer, a
reinforcement, and a thermosetting resin, or plurality of such
prepregs to obtain a prepreg layer or a copper clad laminate. By
alternating layers of prepregs comprising a thermosetting resin and
layers of prepreg comprising only reinforced fluoropolymer
(PTFE-fiberglass), lower cost can be obtained in addition to a
reduced dissipation factor.
[0014] Unexamined Japanese Patent Application 6-322157 suggests the
use of filler materials to achieve a high dielectric constant
composite comprising a narrowly defined epoxy resin, a
reinforcement, and high dielectric constant fillers. 100 to 600 phr
of high dielectric ceramic powder combined with 100 phr of a
polysulfone is used to impregnate a reinforcement. Dielectric
constants>7.4 are taught. This prior art does anticipate the
addition of high dielectric constant ceramics to a layer of epoxy
resin but does not disclose a solution for a low loss laminate or
prepreg, the lowest dissipation factor cited for the compositions
cited as 0.011.
[0015] The present invention anticipates a composition comprising a
fluoropolymer, a reinforcement, and a thermosetting layer that is
capable of producing a laminate with a dissipation factor less than
0.005 and a dielectric constant less than 4.0. It has been
unexpectedly found that one can influence the ability of the
PTFE-fiberglass layer to conform around copper circuitry by
designing a thermosetting adhesive composition of very low
viscosity. The combination of a thermosetting resin, a
reinforcement, and a fluoropolymer is referred to as the low loss
substrate. This low loss substrate or plurality of substrates can
be combined with another low loss substrate such as
PTFE-fiberglass, fluoropolymer film, or FR-4 epoxy-fiberglass
prepreg in any combination or plurality of combinations, or a
reinforcement such as fiberglass that may or may not comprise a
thermoplastic or thermosetting resin, to yield a prepreg layer or a
copper clad laminate.
SUMMARY OF THE INVENTION
[0016] In one aspect, the present invention relates to a process
for fabricating a low loss multilayer printed circuit board using a
bonding ply comprising a fluoropolymer substrate and a
thermosetting adhesive composition. The fluoropolymer composite
comprises at least one fluoropolymer and a substrate selected from
fluoropolymer impregnated woven fabrics, nonwoven fabrics and
polymeric films. The resulting electrical properties have a
dissipation factor less than 0.005 and a dielectric constant less
than 4.0.
[0017] In a further embodiment, the invention relates to a low loss
bonding ply that unexpectedly reforms during lamination to conform
to the outline of print and etched circuitry such that less mass of
thermosetting resin is required to fill gaps in copper circuitry.
This ability to reform during lamination is enabled by a high
flowing thermosetting resin system. This ability to conform to the
outline of circuitry is not limited to the thermosetting resin
system. The PTFE-fiberglass layer unexpectedly participates in gap
filling by reforming around circuitry.
[0018] In another aspect, the invention relates to a multilayer
printed circuit board comprising a plurality of printed circuit
board layers bonded together by means of the same bonding ply.
[0019] In yet another aspect, the invention relates to a
composition comprising a fluoropolymer composite comprising at
least one fluoropolymer and a substrate selected from fluoropolymer
impregnated woven fabrics, nonwoven fabrics and polymeric films;
and a thermosetting adhesive composition that is combined to form a
prepreg or a copper clad laminate.
[0020] In another aspect, the invention comprises any combination
of a low loss substrate containing a fluoropolymer and a
reinforcement with a thermosetting resin that may or may not be
reinforced to form a prepreg or a copper clad laminate.
[0021] In another embodiment, the invention further comprises
combining a prepreg that could be a low loss substrate such as
PTFE-fiberglass, a polymeric film such as skived PTFE, a low loss
substrate such as PTFE-fiberglass-thermosetting resin system, or a
high loss substrate or prepreg such as FR-4 epoxy-fiberglass to
form a copper clad laminate or a prepreg consisting of any one of
the above or a plurality of the above to form a multilayered
prepreg serving as one prepreg or to form a copper clad
laminate.
[0022] In a further aspect, any one of the low loss or high loss
substrates alone or in combination may have some level of porosity
due to laser ablation and be combined in any fashion as above to
form a combined prepreg layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1. Schematic of a low loss substrate comprising a
fluoropolymer, PTFE, a reinforcement, woven Fiberglass, a
thermosetting resin, useful for the low temperature lamination of
print and etched inner layers to make a multilayer circuit
board.
[0024] FIG. 2. A four Layer Hybrid Printed Circuit Board
Manufactured using a Low Loss Bonding Prepreg comprising PTFE, 7628
Style Fiberglass, and a Thermosetting Resin.
[0025] FIG. 3, A four Layer Hybrid Printed Circuit Board
Manufactured using a Low Loss Substrate Comprising PTFE, 106 Style
Fiberglass, and a Thermosetting Resin.
[0026] FIG. 4. Schematic of a PWB core laminate comprising a PTFE
Coated Fiberglass Composite having a Thermosetting Resin Surface
Coating for Low Temperature Lamination and Copper Cladding
[0027] FIG. 5. Schematic of either a copper clad laminate or a
prepreg layer comprising a fluoropolymer, a reinforcement, and a
thermosetting resin, that is laminated together with a
reinforcement, that may comprise a thermoset or thermoplastic, and
either copper foil for the preparation of copper clad laminate, or
laminated with print and etched inner layers during the preparation
of a multilayer printed circuit board
[0028] FIG. 6. Schematic of a copper clad laminate comprising
fiberglass, PTFE, a thermosetting resin adhesive layer, and copper
cladding, where the copper clad laminate has been produced by a
process combining a low loss prepreg substrate comprising PTFE,
fiberglass, a thermoset, or plurality thereof, with one or more of
a low loss substrate comprising a fluoropolymer and a
reinforcement.
[0029] FIG. 7. Schematic of a copper clad laminate comprising
fiberglass, PTFE, a thermosetting resin adhesive layer, and copper
cladding, where the copper clad laminate has been produced by a
process combining a low loss prepreg substrate comprising PTFE,
fiberglass, a thermoset, or plurality thereof, with one or more of
a low loss substrate comprising a fluoropolymer.
[0030] FIG. 8. Schematic of a prepreg material comprising a
PTFE-fiberglass low loss substrate with controlled porosity that
has been filled with a thermosetting resin.
[0031] FIG. 9. A low loss substrate comprising a thermosetting
adhesive and a fluoropolymer impregnated woven fiberglass, that is
used to bond together inner layers that are populated with 1 and 2
oz copper traces. 9A shows a photomicrograph where one ply of
bonding ply has lead to voiding. 9B shows a photomicrograph where
two bonding plies have not overcome this problem of voiding.
[0032] FIG. 10. A low loss substrate comprising a thermosetting
adhesive and a fluoropolymer impregnated woven fiberglass, that is
used to prepare two separate laminates. Laminates were built from a
plurality of low loss substrates comprising a thermosetting
adhesive having either a 25 wt % or a 35 wt % component of the
thermosetting adhesive.
[0033] FIG. 11. A low loss "conformal" bonding ply substrate used
to bond together 2 FR-4 based print and etched inner layers.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The present invention relates to a process for fabricating a
low loss multilayer printed circuit board using a laminate or a
prepreg with a dissipation factor less than 0.005 and a dielectric
constant less than or equal to 4.0. The process comprises
laminating together a plurality of printed circuit board layers by
means of at least one thermosetting adhesive-coated fluoropolymer
composite bonding ply. The bonding ply comprises a fluoropolymer
composite and a thermosetting adhesive composition; the
fluoropolymer composite comprises at least one fluoropolymer and a
substrate selected from fluoropolymer impregnated woven fabrics,
nonwoven fabrics and films.
[0035] PTFE copper clad laminate suppliers currently sell
composites consisting of PTFE and either fiberglass or chopped
fiber. Woven fiberglass is preferably coated with PTFE at
700.degree. F. (371 .degree. C.) to a desired thickness. Generally,
multiple coating passes are necessary to sequentially build layers
of PTFE such that a composite is obtained having the desired
thickness. The coated fiberglass is then sandwiched between copper
to form a composite consisting of a component that is electrically
conductive (the copper) and a component that is not
(PTFE/fiberglass).
[0036] Disclosed herein is a process that enables circuit board
fabricators to connect multiple layers of low loss substrates,
print and etched inner layers of any resin type, together at
reasonable fabrication temperatures (.congruent.350.degree.
F./177.degree. C.). One embodiment of the invention disclosed
herein is a hybrid composite that has the advantages of PTFE but
can be processed like a low temperature thermoset. FIG. 1 shows a
low signal loss microwave circuit bonding ply that can be used to
connect two double sided circuit boards (PTFE/fiberglass double
sided circuits). The thermosetting resin (adhesive) can be thought
of as the adhesive that holds the printed circuit boards together.
During epoxy lamination, for example, the printed circuit boards
can be withdrawn from a press rapidly after holding at their curing
temperature and placed in a cooling press.
[0037] The reinforced first phase (fluoropolymer composite) can be
prepared by impregnating, for example, woven fiberglass in a roll
to roll fashion using a dip-coating process or a dual reverse roll
coating process. Sequential buildup facilitates the manufacturing
of the overall composite. Woven glass fabric is conveniently
impregnated with PTFE dispersion or a common fluoropolymer aqueous
dispersion in a multi-pass process to a desired thickness or build
weight. Coating is continued until a homogenous sheet is formed
where the glass fabric may or may not be completely coated.
[0038] When the reinforcement is woven fiberglass or nonwoven
fiberglass and the low loss resin is a fluoropolymer such as PTFE,
it is preferred that the initial coating of the reinforcement occur
by depositing a silane containing PTFE dispersion onto a silane
treated reinforcement. One embodiment of this invention is the need
to obtain good adhesion between the reinforcement (fiberglass) and
the low loss resin (PTFE). A lack of adhesion between the two
components will lead poor peel strengths, blistering during thermal
excursions during the preparation of printed circuit boards,
mechanical separation of the laminates during routing, and
separation around drilled plated through holes that manifest itself
as white halos. Because the invention described herein comprises a
fluoropolymer and a reinforcement that are exposed to only modest
lamination temperatures, temperatures necessary for the preparation
of conventional thermosets such as epoxy, 170.degree. C. for
example, it cannot be anticipated that the PTFE can be pressed into
the fiberglass such that no voids occur. Because PTFE does not
reach its gel temperature until 320.degree. C. and because it is a
resin that does not flow in the sense of a conventional injection
moldable thermoplastic, it fuses, there must be special attention
to the adhesion of the fluoropolymer to the reinforcement. It
cannot be anticipated that the PTFE is compressed into the smallest
spaces within the fiberglass microfilaments at 170.degree. C.,
conventional epoxy fabrication temperatures, when in fact PTFE is
conventionally pressed into fiberglass near its gel temperature of
325.degree. C. by PTFE-fiberglass laminate suppliers. For these
reasons, it is a preferred embodiment that the first deposition of
the fluoropolymer be conducted at low enough viscosities to obtain
good adhesion of the resin to the reinforcement. It is preferred
that if a waterborne fluoropolymer is used such as PTFE, that the
viscosity of the initial coating be such so as to not compromise
the initial impregnation into the reinforcement. It is anticipated
that a viscosity greater than 20 cp would probably start to
compromise this impregnation, with viscosities greater than 100 cp
very likely to compromise the adhesion of the resin to the
fiberglass. For these reasons, it is preferred that the
fluoropolymer is deposited onto the reinforcement in a layered
fashion, such that the first layer is a filler-free fluoropolymer
composition, that is, it contains no ceramic dielectric modifiers
or other fillers that would impact the adhesion to the
reinforcement, followed by subsequent layers that may or may not
contain ceramics to modify the electrical properties. Subsequently
deposited layers may contain a finely divided ceramic material
distributed in a fluoropolymer composition.
[0039] Impregnation of the reinforcement is critical to the
survival of the laminate during the many processing steps in a
printed circuit board shop. This is largely dependent on the first
impregnation of the reinforcement. One embodiment of this invention
is the use of a melt flowing fluoropolymer during the first
impregnation pass of the reinforcement. This could include
copolymers of PTFE and perfluorinated methylvinylether or
perfluorinated propylvinylether. It is preferred that this melt
flowing fluoropolymer be combined with a silane coupling agent to
resist wet chemical migration and improve the adhesion to the
reinforcement. It is preferred that the melt flowing fluoropolymer
has a melting point at least at or around 300.degree. C. such that
it does not flow during hot air solder leveling or sweat bonding.
It should have a high melting point and a low enough melt viscosity
to flow into the fiberglass.
[0040] One large deficit of the Unexamined Japanese Patent
Application 6-322157 is that it teaches applying a ceramic modified
polysulfone varnish to a reinforcement. 100 phr of thermoplastic to
100 to 600 phr of ceramic modifier is suggested. 100 to 600 phr of
a high dielectric constant ceramic modifier combined with 100 phr
of PTFE would not impregnate many reinforcements such as 1080 type
fiberglass because the resin viscosity would be unacceptably high
to fill the spaces between 5 micron microfilaments. PTFE is not a
melt flowing fluoropolymer. It will compress to some degree and
fuse but it does not flow. An injection moldable grade polysulfone
would be expected to flow better into the small cavities in a
finely woven fiberglass such as 1080, however, this may not occur
if the polysulfone is highly loaded with ceramic, the viscosity is
very high, and the ceramic particles are of such size to prevent
impregnation into the fiberglass. Because the scope of the
invention is to conduct lamination at epoxy temperatures, poor
impregnation could not be overcome with low lamination
temperatures, epoxy processing temperatures of 170.degree. C. being
well removed from PTFE fusing temperature of 320.degree. C.
6-322157 anticipates the problems of poor adhesion, void formation,
and lack of flow, yet it is suggested that a porous state is
acceptable as long as the ceramic modifier does not fall out and
separate from the glass cloth. PTFE based printed circuit board
composites having voiding at the reinforcement level is not
acceptable. The process of drilling a printed circuit board exposes
the resin-to-reinforcement interface. The wet chemicals used for
the treatment of drilled holes and for subsequent copper deposition
then wick into voided areas and are encapsulated within the printed
circuit board during copper plating of the plated through hole.
This condition leads to blistering during thermal excursion as
volatiles rush to escape during solder reflow, for example, or
solder mask curing. These problems are overcome by impregnating a
reinforcement first with a PTFE dispersion that has a very low
viscosity, 20 cp for example, that additionally contains a coupling
agent for adhesion to the reinforcement. Low levels of ceramic
could possibly be tolerated if they did not result in an
appreciable increase in viscosity and if they did not retard the
impregnation of the reinforcement.
[0041] A fluoropolymer impregnated woven fabric comprises a woven
fabric that has been lightly impregnated with a fluoropolymer layer
to insulate the fiberglass layer from any subsequent fluoropolymer
coating layer that may contain ceramic that would lead to voiding,
poor adhesion to the fiberglass, blistering, or haloing during
drilling. While not preferred, this fluoropolymer impregnated woven
fabric could contain modest quantities of ceramic only so long as
it did not compromise the properties listed previously.
[0042] It is a preferred embodiment of this invention that the
fluoropolymer resin be cast onto a substrate. The substrate may or
may not be a reinforcement. Suitable substrates include: woven or
non-woven fabrics; crossplies of unidirectional tape; a polymeric
film; or a metallic film. Metallic films include copper, aluminum,
and the various grades of steel. Polymeric films include
Kapton.RTM. (available from Dupont), and Upilex.RTM. (available
from UBE industries), a polyimide based on biphenyltetracarboxylic
dianhydride and either of p-phenylenediamine or 4,4'
diaminodiphenylether. Woven fabrics can be prepared from glass
filaments or filaments based on various polymers. Suitable organic
polymeric fibers consist of the following: PTFE or other
fluoropolymer fibers; polyaramides such as Teijin's Technora based
on p-phenylenediamine and 3,4'-diaminodiphenylether, meta aramids
such as Nomex.RTM. based on poly(m-phenyleneisophthalamide); liquid
crystalline polyesters such as those based on hydroxynapthoic acid
and hydroxybenzoic acid; polyetheretherketones (PEEK.RTM.,
available from Victrex USA); polybenzoxazole (PBO, available from
Toyobo); and polyimides. These polymeric fibers can be used to make
woven fabrics or they can be chopped or pulped and used to make
non-woven fabrics. In the preparation of non-woven fabrics, blends
of different fibers might be used, or blends containing chopped
glass fiber can be used. Non-woven fabric has the advantage that
very thin laminates can be prepared. Because the fibers are random,
improved drilled holes can be obtained, regardless of the drilling
technique, laser or mechanical. Low in-plane CTE results in
exceptional layer to layer registration. The non-woven fabric can
be coated roll to roll in a typical dipcoating process or
alternatively staple-pulped fiber can be added to an aqueous PTFE
dispersion and coated onto a release substrate. In another
embodiment, a fluoropolymer coating can be applied to the fabric by
hot roll laminating a fluoropolymer film or a fluoropolymer skived
material into the fabric thus eliminating the need for multiple
coating passes. The film may or may not contain a ceramic
dielectric modifier.
[0043] Woven glass reinforced composites could be prepared from the
following glass styles (E, D, S, NE), or mixtures thereof. Newly
developed NE glass styles available from Nittobo (Japan) have lower
loss characteristics but have a cost disadvantage. Glass fabric
based on 4-6 micron filaments is preferred from a drilling
perspective. Typical glass styles that are preferred include: 106,
1080, 2112, 2113, 2116, and 7628. For laser drilling applications
the smaller diameter based glass fabrics are preferred. The
fiberglass must be largely free of sizing agents used to weave
fiberglass. Secondly, the fiberglass must be treated with a silane
coupling agent to resist wet chemical migration and to improve the
adhesion of the fiberglass reinforcement to the silane containing
first impregnation pass of fluoropolymer. A lack of silane will
lead to defective areas where the low loss substrate has been
mechanically drilled as indicated by white halos around the holes.
For these reasons standard PTFE impregnated fiberglass that might
be used in industrial applications is not suitable for electrical
laminate applications.
[0044] Flat glasses are woven fiberglasses derived from low twist
or zero twist yarns. In the weaving process, yarn bundles are
typically twisted such that they can be readily woven without the
bundles losing their integrity. Generally the warp yarns are pulled
under tension through a device and the fill yarns are inserted
across the rows of warp yarns using a rapier or air jet loom, for
example. Low twist yarns have straighter filaments than can be more
readily flattened. The fiberglass can be prepared by starting with
zero or low twist yarns that may or may not be somewhat flat or
they can be flattened in a post weaving process where the yarns are
mechanically flattened or the yarns can be flattened due to an
impinging spray.
[0045] The embodiment of this invention can be envisioned or
modeled like a resin coated copper system. In this invention, a
given amount of resin is disposed over the surface of a flat
fluoropolymer substrate. The thermosetting component of the
invention is expected to fill the gaps created by the manufacture
of interlayer circuitry. It is known from resin coated foils, that
a given amount of resin is necessary to fill certain types of
copper circuitry. It is believed that 35 microns of flowable resin
in a resin coated copper approach is necessary to fill 1 oz copper
(36 microns) and that 50 microns of flowable resin is necessary to
fill 2 oz copper (72 microns). A lack of resin will lead to voiding
as shown in FIG. 9. To the extent that the fluoropolymer substrate
is not flat, resin is wasted filling those cavities. Because the
flexible PTFE coated fiberglass has the ability to conform to the
contour of innerlayer circuitry (innerlayer is the correction
terminology), this invention has the advantage that less resin is
required to fill copper gaps. For these reasons the viscosity of
the thermosetting adhesive over the temperature range used for
pressing by a pwb fabricator is critical. If the thermosetting
adhesive has a high viscosity voiding will occur as shown in FIG.
9. For this reason it is critical that the thermosetting adhesive
have a melt viscosity less than 100 Pa*s over the temperature range
used for pressing (room temperature to 200.degree. C.). Higher
viscosities can be overcome by application of high pressures during
lamination. However most pwb fabricators do not have lamination
presses that exceed 500 psi over a large platen size such as a
36".times.48" footprint. Therefore, to move adhesive from areas
where it is not needed to areas where it is needed, to fill gaps
between copper circuitry, the melt viscosity must be less than 100
Pa*s for a long enough time to eliminate voiding. A melt viscosity
less than 100 Pa*s (5 rad/s, 5% strain) over the lamination
temperatures of interest will be used to describe a high flowing
thermosetting resin system.
[0046] A high flowing thermosetting resin system also enables the
PTFE-fiberglass to conform to the outline of copper circuitry. A
low flowing formulation of high viscosity will more readily lead to
a final composition where the PTFE-fiberglass layer is fairly flat
when disposed between opposing layers of print and etched copper
circuitry. However, when the thermosetting resin is high flowing,
the PTFE-fiberglass layer can reform during lamination such that
the PTFE-fiberglass itself will help fill the gaps in copper
circuitry. FIG. 11 shows bundles of PTFE coated fiberglass that
have moved into the gaps between the copper circuitry. This allows
the movement of the thermosetting resin from areas of high pressure
to areas of low pressure. FIG. 11 also shows a bonding prepreg
layer where the thermosetting layer has flowed away from the copper
trace such that the PTFE-fiberglass layer is disposed directly on
top of the copper trace and the high flowing thermosetting resin
has moved between the copper circuitry. This degree of
"reformability" of the PTFE-fiberglass layer is promoted by a high
flowing thermosetting resin system that consists predominantly of
low molecular weight resins, preferably non advanced and not highly
polyfunctional such that a high crosslink density is achieved
quickly, as indicated by a low gel time, such that a wide
processing window is available before the rapid build up of
viscosity.
[0047] The viscosity of the thermosetting adhesive over the
lamination temperature has a second critical function.
Polytetrafluoroethylene and related fluoropolymers are some of the
lowest loss materials available in the marketplace today. Other
thermosetting resin systems have to be considered higher loss
materials. This is true because non fluorinated materials must be
flame retarded resulting in an elevation of the dissipation factor.
Therefore, thermosetting adhesive formulations that have very low
viscosity and high flow can be expected to flow out of the
laminate. It is an unexpected benefit when the highest loss
component, the thermoset, flows out of the laminate. It is only
desirable to leave the minimum amount of thermoset possible to fill
gaps in copper circuitry.
[0048] This can be obtained by optimizing the following variables:
the gel time of the resin, the degree of advancement of the resin
during B-staging, and the viscosity of the thermoset as it cures.
The gel tine can be manipulated by decreasing the concentration of
chemical groups crosslinking, by eliminating the use of
multifunctional chemical groups such as trifunctional epoxies or
phenol novolacs that are somewhat advanced by nature, by
eliminating or limiting the use of chain extended epoxies or chain
extended moieties of any chemical nature, by varying or reducing
the catalyst concentration, and by adding reactive diluents that
are known to increase the gel tine of a thermoset. Alkyl
substituted mono glycidyl ethers (C8-C14, Heloxy Modifier 7 or 8
from Resolution Performance Products) are monofunctional epoxies
that reduce thermoset crosslink density and lead to a slower cure
rate. Phenyl glycidyl ether is another example. Polyfunctional
alkyl substituted glycidyl ethers (Heloxy Modifiers 84, a
trifunctional glycidyl ether based on an aliphatic triol) are less
effective yet they result in less of a compromise. Epoxies based on
aliphatic alcohols are less reactive than epoxies based on aromatic
alcohols. Modifiers that increase the total mass relative to the
epoxy, the mass or equivalent weight per epoxy, reduce the reaction
rate of the epoxy. Simply, the lower the chemical concentration the
longer the gel time and the longer the time the thermoset will
spend at a low viscosity before curing.
[0049] The amount of mass of thermosetting resin is critical. Each
side of the low loss substrate must contain a minimum of 0.25
lbs/yd.sup.2 of adhesive to fill 0.5 oz circuitry to 0.75
lbs/yd.sup.2 to fill 2 oz circuitry. This will vary a little bit
depending on the design, the density of the traces involved, the
spacing of the traces to each other, whether or not there are
stacked layers of opposing traces over a very thick multilayer
printed circuit board, 16 layers for example. If a 16 layer circuit
board is prepared and each layer contains traces of 1 oz circuitry
in the same spots and voids in the same spots, conceivably there
could be 20 mils extra of copper thickness in one location leading
to an area of high pressure during lamination, while the gap areas
have 20 mil less of mass leading to a low pressure zone, the result
being an area of very little pressure if any to push thermosetting
adhesive into the cavities. For these reasons it is critical that
enough thermosetting resin is available to fill a given volume
between traces and that the thermosetting adhesive has a low enough
viscosity for a long enough time to move from areas of high
pressure to low pressure.
[0050] Woven glass fabrics are particularly suitable as substrates
for the fluoropolymer composite. Examples of such woven glass
include 7628, 1080, or 106 style glasses with a 508 heat cleaned
finish produced by Hexcel Schwebel.
[0051] One further embodiment of the invention is that the
fluoropolymer coated fiberglass, the low loss substrate, might be
perforated before application of the thermosetting resin. One
disadvantage of the original embodiment is that the thermosetting
resin is not able to flow through the uniform layer of
PTFE/fiberglass. For this reason, thermosetting resin that might be
disposed above the PTFE/fiberglass sheet is not available to fill
gaps between copper traces in a layer located below the
PTFE/fiberglass. In the ideal state, all of the thermosetting resin
that is coated onto both sides of the PTFE/fiberglass is present to
fill the gaps in the copper traces created by the print and etch
process. A schematic of the embodiment is shown in FIG. 8.
Perforation of the PTFE/fiberglass sheet can be achieved using a
mechanical cutting method or a laser drilling process. Although
FIG. 8 shows the laser drilled holes between fiberglass bundles, it
is envisioned that the laser drilled holes would be spaced in such
a way to achieve good copper trace gap filling. Therefore the laser
drilled holes would not necessarily conform to any features of the
fiberglass but rather to a drilling pattern necessary to create a
permeable enough membrane to allow thermosetting resin to readily
flow from side to side. It is envisioned that while FIG. 8
represents a rather uniform hole, it is recognized that a laser
drilled hole or a mechanically drilled hole would not be perfectly
cylindrical, nor would the holes appear similar because each
drilled hole would be created from a potentially minutely different
fiberglass topography. Using this embodiment of the invention, a
plurality of fluoropolymer coatings would be necessary to
impregnate the fiberglass, followed by a step to create the via
holes, followed by a step to surface treat the via to get good
adhesion between the via hole wall and the thermosetting resin that
will be subsequently coated onto the surface and into the via
holes. Alternatively, fiberglass could be lightly coated with PTFE,
the substrate perforated, followed by further coatings of PTFE that
do not successfully close the holes, followed by a subsequent
deposition of a thermosetting resin.
[0052] The advantage of this improvement is that it allows the
creation of a fill layer where more of the mass can be considered
useful for gap filling. This is particularly advantageous for
thinner substrates, from 1-3 mil, where thick building blocks of
two thermosetting layers separated by a PTFE/fiberglass layer would
render the product too thick to be appropriate. In the ideal state,
the entire mass would be available for gap filling/conformal
filling. However, if enough of the woven fiberglass structure is
retained to yield some level of a continuous reinforcement, a
higher degree of controlled dimensional movement can be achieved
during standard printed circuit board fabrication steps.
[0053] Various fluoropolymers can be used to prepare the reinforced
first phase. Polytetrafluoroethylene (PTFE) or modified
polytetrafluoroethylene is well known to those skilled in the art.
Modified PTFE contains from 0.01% to 15% of a comonomer which
enable the particles to fuse better into a continuous film. PTFE is
typically modified with a small quantity of a fluorinated alkyl
vinyl ether, vinylidene fluoride, hexafluoropropylene,
chlorotrifluoroethylene, and the like. High level of modification
leads to polymers such as PFA poly(perfluorinatedalkylvinyle-
ther-tetrafluoroethylene) or FEP
poly(perfluorinatedtetrafluoroethylene-he- xafluoropropylene).
Other fluoropolymers which may serve as a dielectric include:
polychlorotrifluoroethylene; copolymers of chlorotrifluoroethylene
with vinylidene fluoride, ethylene, tetrafluoroethylene, and the
like; polyvinylfluoride; polyvinylidenefluoride; and copolymers or
terpolymers of vinylidene fluoride with TFE, HFP, and the like; and
copolymers containing fluorinated alkylvinylethers. Other
fluorinated, nonfluorinated, or partially fluorinated monomers that
might be used to manufacture a copolymer or terpolymer with the
previously described monomers might include: perfluorinated
dioxozoles or alkyl substituted dioxozoles; perfluorinated or
partially fluorinated butadienes; vinylesters; alkylvinyl ethers;
and the like. Hydrogenated fluorocarbons from C2-C8 are also
envisioned. These would include trifluoroethylene,
hexafluoroisobutene, and the like. Fluoroelastomers including the
following are also envisioned: HFP with VDF; HFP, VDF, TFE
copolymers; TFE-perfluorinated alkylvinylether copolymers; TFE
copolymers with hydrocarbon comonomers such as propylene; and TFE,
propylene, and vinylidene fluoride terpolymers. Fluoroelastomers
can be cured using the following crosslinking agents: diamines
(hexamethylenediamine); a bisphenol cure system
(hexafluoroisopropylidenediphenol); peroxide
(2,5-dimethyl-2,5-dit-butyl-peroxyhexane); any base that can act as
a dinucleophile.
[0054] Fluoropolymer dispersions that (1) readily rewet (2) are
available at low cost and (3) have low dielectric loss
characteristics are preferred. Aqueous dispersions of
fluoropolymers can contain a particle size from 1 nanometer to 1000
nanometers. The particle size of the fluoropolymer dispersion is
not important as long as the substrate can be well impregnated.
Microemulsions or blends of conventional fluoropolymer dispersions
with aqueous microemulsions are also suitable. The fluoropolymer
component could also be coated from a solvent vehicle onto the
reinforcement.
[0055] Although it is envisioned that the reinforced low loss
substrate component comprise a fluoropolymer, the low loss
substrate could be comprised of a hydrocarbon polymer that could
benefit from an adhesive layer that could improve the performance
of the hydrocarbon resin alone. Cured elastomers such as
polybutadiene, for example, are known to have poor adhesive
properties and would benefit from a second layer of an adhesive
phase. The rubber may be any natural or synthetic rubber or a
combination thereof. Generally, the rubber may be any saturated or
unsaturated polyalkylene rubber made up of ethylene, one or more
alkenes with 3-8 carbon atoms, for instance, propylene and/or
1-butene, and, if desired, one or more polyethylenically
unsaturated compounds, for instance, 1,4-hexadiene,
dicyclopentadiene, 5-methylene-2-norbornene,
5-ethylidene-2-norbornene and 5-isopropylidene-2-norbornene. Any
rubbers containing the following monomers would be anticipated:
butadiene, isoprene, styrene, divinylbenzene, maleic anhydride,
anhydride functionalized butadienes or other aliphatic or aromatic
dienes. The rubber can therefore be any suitable natural rubber,
synthetic polyisoprene, any of the neoprenes, (polychloroprene),
styrene-butadiene rubbers (SBR), styrene-butadiene-divinylbenzene
available from Ricon, acrylonitrile-butadiene rubbers (NBR),
acrylonitrile-butadiene-styrene polymers (ABS) high molecular
weight olefin polymers with or without other monomers or polymers
such as butyl rubber and cis- and trans-polybutadienes, bromobutyl
rubber, chlorobutyl rubber, ethylene propylene rubbers, nitrile
elastomers, polyacrylic rubber, polysulfide polymers, silicone
elastomers, poly- and copolyesters, ethylene acrylic elastomers,
vinylacetate ethylene copolymers, or chlorinated or
chlorosulfonated polyethylenes, or a mixtures thereof. The rubber
may also contain a ceramic modifier.
[0056] It is envisioned that the low loss substrate could comprise
a ceramic, fused silica for example, a reinforcement, woven
fiberglass for example, and a cured elastomer resin system,
containing high and low molecular weight polymers comprised of
butadiene, isoprene, maleic anhydride, anhydride functionalized
butadiene or other dienes, divinylbenzene, nobornene, neoprene, or
styrene. The second adhesive component then might comprise a
polymeric resin system know to have better adhesive properties or
better flow properties. This second adhesive component might
include an epoxy, a cyanate ester, or any number of the various
thermosetting resin systems known to those skilled in the art.
[0057] The surface of the fluoropolymer composite or the first
phase composite can then be treated before applying the
thermosetting adhesive composition to facilitate bonding
therebetween. Etching techniques for modifying the surface of a
fluoropolymer are known in the art. These include etching by:
sodium ammonia etch, radiation, electron beam, sodium naphthalene
etch, plasma using hydrogen, argon, nitrogen, carbon tetrafluoride
gases, corona, vacuum discharge, and the like. Once the surface of
the first phase is treated, a thermosetting resin can be applied by
conventional coating methods. Depending on the amount of ceramic
incorporated into the reinforced fluoropolymer substrate, surface
treatment may not be necessary to obtain good adhesion to the
adhesive layer. It is known to those skilled in the art that
laminates having high ceramic loadings that have drilled holes may
not need surface treatment before plating. The thermosetting resin
is typically coated simultaneously onto both sides of the
fluoropolymer composite using two reverse roll treaters, one per
side. This can also be accomplished using wrapped wire wound rods.
The thermosetting resin is typically prepared by driving off the
solvents used to dissolve the thermosetting resin. The
thermosetting resin is applied as a flat continuous film on the
surface of the fluoropolymer impregnated reinforced sheet. Although
it is preferred that the second component be a thermosetting resin
processable at low temperatures, deposition of a thermoplastic
layer onto the first component is also envisioned.
[0058] The thermosetting component is preferably a
non-fluoropolymer but a thermosetting fluoropolymer is also
envisioned. The thermosetting adhesive component should have a
glass transition of at least 100.degree. C. or a glass transition
that is very difficult to detect by common techniques such as DMA
or TMA, or a CTE that results in a total (x, y, and z) coefficient
of thermal expansion in the unreinforced state of around 50-100
ppm/.degree. C. over the temperature range 50-300.degree. C.,
although less than 50 ppm/.degree. C. would be preferred.
[0059] Typical thermosetting resin systems that could be used
include: epoxies (phenol epoxy novolacs; cyclopentadiene based
epoxies; brominated epoxies; diamine cured epoxy resin systems
(diaminodiphenylsulfone); trisepoxies; multifunctional epoxies;
styrene-maleic anhydride copolymers cured with epoxies or
polyamines; norbornene-maleic anhydride copolymers cured with
epoxies or polyamines; bicyclic alkane compounds of the general
structure bicyclo[x.y.z.]alkane-anhydride copolymers cured with
epoxies; cyanate ester resins such as those based on bisphenol A or
novolac resins; cyanate ester resins cured with epoxies;
polynorbornene cured with a free radical generator; polynorbornene
blends containing, for example, any combination of polybutadiene or
polyisoprene; free radically cured polybutadiene of varying
molecular weights with optionally polyisoprene; acetylene
functionalized polyimide; functionalized polyphenylene oxide and
blends of functionalized PPO with epoxies; bis-triazine resin
systems with the optional addition of epoxies; multifunctional
aziridines; poly(bismaleimides), specifically bismaleimides based
on diaminodiphenylmethane; and bismaleimides cured with
diallylbisphenol A or other bisallylphenyl compounds; norbornene
terminated polyimides, poly(bis phenylcyclobutane); free radically
cured unsaturated polyesters; and the like. Other non fluorinated
adhesive layers could also include: polymethylvinylether;
polyvinylpyrrolidone; polybutadiene; copolymers of polybutadiene
and styrene; elastomers containing any combinations of butadiene,
isoprene, styrene, or neoprene, divinylbenzene, anhydride
functionalized butadienes, maleic andhydride, elastomers of
ethylene and propylene; elastomers of acrylonitrile and butadiene;
elastomers of butadiene-styrene-divinylbenzene, and the like. The
thermosetting resin system does not include lightly crosslinked
pressure sensitive adhesives which have a low glass transition
temperature.
[0060] The adhesive layer could also include a fluorelastomer.
Fluoroelastomers include the following: copolymers of vinylidene
fluoride and hexafluoropropylene; copolymers of
hexafluoropropylene, vinylidene fluoride, and tetrafluoroethylene;
copolymers of vinylidene fluoride and perfluoroalkyl vinylethers
with or without tetrafluoroethylene; copolymers of
tetrafluoroethylene with propylene; copolymers of
tetrafluoroethylene with perfluoroalkylvinylethers; a terpolymer of
propylene, vinylidene fluoride, and tetrafluoroethylene.
Fluoroelastomers can be cured using the following crosslinking
agents: diamines (hexamethylenediamine); a bisphenol cure system
(hexafluoroisopropylidene- diphenol); peroxide
(2,5-dimethyl-2,5-dit-butyl-peroxyhexane); any base that can act as
a dinucleophile. In some cases it might be preferred to incorporate
a cure site monomer into the polymer backbone to promote curing.
These might include halogen-containing olefins such as
1-bromo-2,2difluoroethylene or 4-bromo-3,3,4,4-tetrafluoro-butene.
Other cure site monomers might include nitrile containing
vinylethers and hydrogen containing olefins.
[0061] Either the thermosetting resin adhesive component or the
reinforced low loss substrate (PTFE-fiberglass) or both may
optionally contain any one or combination of a number of ceramic
dielectric constant modifiers. These ceramic modifiers are
typically polymeric, inorganic, ceramic, or organometallic. Ceramic
modifiers are used to modify the electrical, thermal, improve the
dimensional stability of the laminate, and reduce cost. It is well
known that the addition of various ceramics will reduce the
coefficient of thermal expansion of the composite, a reduction in
the z axis being highly desirable for the reliability of plated
through holes. X-Y CTE reduction enables smaller copper pads to be
used and less layer to layer misregistration.
[0062] In many cases, ceramics are added to tailor the dielectric
constant of the composite. Ceramics typically include: quartz,
alumina, titanium dioxide, strontium titanate, barium titanate,
alumina, fused, colloidal, or fused silica, chopped glass fiber,
magnesia, aluminum silicate (kaolin), steatite, zircon, quartz,
boron nitride, silicon nitride, aluminum nitride, silicon carbide,
talc, beryllia, barium titanate, mica, hollow or solid glass
spheres. For a laminate or prepreg having a dielectric constant
less than 4.0, the choice of ceramics is limited. The preferred
ceramics are fused silica, quartz, kaolin, talc, alumina, and
titanium dioxide. Ceramics with a dielectric constant above 4.0 in
the natural state, such as talc, alumina and titanium dioxide would
have to be limited in order to minimize their contribution to an
increased dielectric constant. The preferred embodiment comprises a
dominant proportion of the ceramic to be less than a Dk of 4.0,
fused silica for example, with a minor component of a high
dielectric constant material used only to modify the dielectric
constant slightly. Some high dielectric constant ceramics such as
titanium dioxide have a much lower effective dielectric constant
when used as a particulate powder and combined with other materials
such as PTFE.
[0063] It is also envisioned that the addition of certain polymeric
fillers available as powders could be advantageous. These include:
including PTFE, crosslinked PTFE, available as either a powder or a
dispersion, polyetherketones, polyetheretherketones,
polyphenylenesulfide, polyethersufone, polyimide, polyester, liquid
crystalline polyester, polyamide, polyesteramide, polybutadiene
rubber, and other elastomeric materials which might include
butadiene, isoprene, neoprene, dicyclopentadiene, styrene,
divinylbenzene, and maleic anhydride. Low loss fillers that can be
readily dispersed in a solvent-borne matrix are preferred. For
aqueous dispersions, small particle powders are preferred. PTFE
powders can be dispersed in either of the thermosetting varnish or
the fluoropolymer-fiberglass low loss substrate.
[0064] Polytetrafluoroethylene powders of controlled particle size
are preferred. PTFE has the advantage of very low loss relative to
the inorganic fillers. PTFE has the further advantage of less drill
wear relative to a hard filler such as fused silica. It has been
unexpectedly found that PTFE can be readily dispersed in a
thermosetting resin solution to reduce the resulting loss at 10
GHz. PTFE is a lubricant and is also expected to help lubricate the
bit during drilling. A laminate comprised of PTFE powder, a
thermosetting resin, copper, and a reinforcement is also
envisioned.
[0065] The introduction of particulate ceramic modifiers,
particularly inorganic ceramics, into a printed circuit board
laminate is not without its drawbacks. The ceramics introduce
another interface into the composite that can be a source of
moisture absorption leading to blistering and delamination upon
exposure of the board to wet chemistry steps (print and etch),
followed by exposure to higher temperatures during sweat soldering
or hot air solder leveling. The proximity of the ceramic to the
glass or polymeric reinforcement may lead to a weak boundary region
between interfaces leading to a laminate that is susceptible to
delamination, blistering, or the plating of unwanted metals. In
addition, the ceramics generally do more damage to a mechanical
drill bit during the preparation of plated through holes. This
exposure of new ceramic surface area during mechanical drilling
might also be a source of failure. It has been previously disclosed
in the art to pre-coat the surface of the particulate with a
hydrophobic coating to improve the moisture resistance of the
resulting composite which is also claimed to improve the adhesion
of the particle to the matrix materials. (U.S. Pat. Nos. 5,024,871,
4,849,284, and 5,149,590). The disclosed examples specifically
teach the use of hydrophobic coatings consisting of silanes,
zirconates, or titanates, all well known inorganic coupling
compounds. According to prior art, in a separate step, the
particulate is precoated and then the precoated particle can be
further formulated, extruded, or further processed in a separate
step. Depending on whether the ceramic is added to a waterborne
system or a solvent-borne thermosetting system, there use of a
hydrophobic surface coating may or may not be required. Other
technologies are available that eliminate the need for a
hydrophobic coating such as an organosilane.
[0066] The surface of the ceramic is readily treated with silanes.
Fused silica, for example, has a high concentration of hydroxyl
groups at the surface of the particle that is dependent on the
milling process. Less than 10 microns sized particles can readily
be obtained in a wet milling process that leaves the surface rich
in hydroxyl functionality. Silane groups can have the following
functionality R.sub.nSiX.sub.(4-n) where X is a leaving group
consisting of alkoxy, acyloxy, amine, an chlorine group, or hydroxy
as an intermediate. The R group can be an alkyl or aromatic
functionality.
[0067] The particle size of ceramic modifiers is preferred to be
less than 25 microns. Particles greater than one micron in size are
harder to disperse, lead to uneven mechanical drilling, and can
potentially protrude through copper cladding.
[0068] The dielectric properties will consequently be a combination
of the thermosetting component and the low loss substrate.
Therefore it is preferred to limit the thermosetting component to
just a sufficient quantity to accomplish bonding of the various
layers. Because an epoxy, for example, has a substantially worse
dielectric loss, it is preferred to limit its use. The preferred
embodiment includes coating a 0.1 to a 1.0 mil dry layer of the
thermosetting resin onto the surface of the fluoropolymer coated
glass composite. Because the thermosetting layer is preferably
thin, inorganic or organic ceramic modifiers incorporated into the
thermoset should have a sufficiently low particle size to yield a
homogeneous film. The ceramic should be less than 50 microns in
size, preferably less than 25 microns, and most preferred, less
than 10 microns. The ceramic should be less than 80% by weight of
the combined filler and thermosetting resin total dry weight,
preferably from 10-60%, and most preferred from 30-60%. Ceramic
concentrations that exceed the volume of the resin necessary to
coat the surface of the ceramic particles and bond to the copper
cladding lead to poor interlaminar adhesion and poor copper
adhesion. Too low a ceramic content leads to less dimensional
stability, higher cost, and in some cases a higher dissipation
factor.
[0069] The thermosetting resin composition and the fluoropolymer
dispersion can be coated using a number of different methods. The
resin compositions can be applied to the carrier or substrate using
spray coating, dipcoating, reverse roll coating, gravure coating,
metering rod coating, pad coating, or any combination of the above.
In the case of the fluoropolymer component, the preferred method is
to dipcoat the reinforcement into a resin composition and using a
metering rod control the amount of pickup of the resin composition
onto the carrier or reinforcement. In the case of the thermosetting
resin system, it can be metered on by a Mayer rod after dipcoating
the fluoropolymer sheet into a resin bath or it can be transferred
coated using a dual reverse roll controlled gap setup.
[0070] The weight ratio of the thermosetting hydrocarbon resin to
the reinforced low loss substrate layer is preferably 1:1 to 1 to
50. It is more preferred that the thermosetting resin have a weight
ratio of 1:2 to 1:20 to the reinforced core. It is most preferred
that the surface coated thermosetting resin have a weight ratio of
1:3 to 1:10. The most preferred ratio of thermosetting resin
surface coating to the reinforced core will vary depending on the
low loss characteristics of the thermosetting adhesive layer, the
thickness of the copper circuitry that needs to be encapsulated,
the degree to which the copper is etched yielding a requirement for
a volume of space that must be filled by the flowing thermosetting
component, and the amount and type of ceramic that may or may not
be added to the thermosetting resin composition. The previously
described weight ratios take into consideration any ceramic filler
that would be added to the thermosetting resin composition.
Although ceramic might also be added to the low loss substrate core
material, the weight ratios of the thermosetting adhesive layer to
the PTFE-fiberglass layer refer to the total weight of the low loss
substrate (PTFE-fiberglass), regardless whether the low loss
substrate is comprised of a resin, a reinforcement, or a ceramic.
The low loss substrate (PTFE-fiberglass layer) could contain up to
80 wt % ceramic modifier.
[0071] This invention also enables the incorporation of UV dyes
into a fluoropolymer based printed circuit board. UV dyes are
typically incorporated into FR-4 epoxy formulations to enable
automated optical inspection (AOI). Many of these dyes would not be
stable at the processing temperatures of many fluoropolymers.
However, the UV dye can be added as an additive to the lower
temperature thermosetting adhesive composition.
[0072] This invention also envisions the addition of alternative
flame retardants. Organic compounds containing phosphorous are
known to be suitable replacements for bromine containing organics.
Triphenyl phosphate and polymer based phosphates would be further
examples.
[0073] In an alternative embodiment of the invention, the bondply
or a plurality of bondplies can be laid up with copper and pressed
in a conventional FR-4 copper clad lamination press. A single bond
ply can be sandwiched between coppers to make a single laminate
core, as shown in FIG. 4. The choice of copper styles could include
rolled or electrodeposited. The copper could be zinc free or zinc
containing, low profile, very low profile, reverse treat, ultralow
profile, or omega foils. Copper could also be sputtered onto the
faces of the adhesive layer to obtain very thin layers of
copper.
[0074] As yet another embodiment, the bondply can be laid up with a
reinforcing layer. This reinforcing layer could include wovens,
nonwovens, or films, as show in FIG. 5. The reinforcing layer could
be resin impregnated or not. If resin impregnated, the resin could
be any one of the thermoplastic or thermosetting resins suggested
for the low loss substrate or the adhesive layer.
[0075] As a separate embodiment the bondply can be laid up with
plies of low loss substrate that have no thermosetting component to
them, as show in FIG. 6. The advantage of this approach is that a
minimum of the thermosetting adhesive is used. The low loss
substrate, PTFE-fiberglass for example, would have to be surface
treated for bonding to the bondply. If an appropriate adhesive
layer were used, such that adhesion to the PTFE could be achieved
without a surface treatment, this would be one added benefit. For
the preparation of a laminate that involved a plurality of bonding
plies and a plurality of low loss substrates it would be desirable
to locate the bondply facing the upper and lower copper layers for
good adhesion. The low loss substrate would then be placed in
alternating layers with the bondply. This approach has the
advantage that half the adhesive is used between a layer of bondply
and low substrate versus two plies of bonding ply that leads to a
thick adhesive layer that is not used for any gap filling. The net
result is less adhesive and a lower dissipation factor. The
dielectric constant would also be reduced by using layers of
PTFE-fiberglass. A further advantage is that half of the low loss
laminate used would not have to be processed into bondply thus
reducing the cost of the laminate.
[0076] A similar strategy could be employed for the preparation of
very low loss laminates. Thermoset treated PTFE-fiberglass could be
intermixed with layers of surface treated skived PTFE film. This
would result in decreased dielectric constant, decreased
dissipation factor, and a reduced cost, when compared to preparing
a laminate of pure PTFE-fiberglass-thermosetting resin.
[0077] Essentially any printed circuit boards may be laminated
together using a plurality of bonding plies or low loss substrates
or reinforcements according to the present invention. In
particular, hybrid printed circuit boards composed of epoxy
fiberglass composites, such as FR-4; or laminates comprised of any
of the following: PTFE; cyanate ester; polyimide; styrene; maleic
anhydride; butadiene; bismaleimide; isoprene; neoprene; polyester,
and others known to those skilled in the art would be suitable.
EXAMPLES
Example 1
Preparation of a Fluoropolymer Coated Woven Glass Fabric
[0078] 7628 style woven fiberglass with a 508 heat cleaned finish
(available from Hexcel Schwebel) was further heat cleaned in a 3
zone vertical coating tower at 7.5 feet/min. The temperatures in
the 3 zones were as follows: 121.degree. C., 204.degree. C., and
418.degree. C. A 5% solution of 3-aminopropyltriethoxysilane in
water was then applied to the fabric using a smooth metering rod.
The fabric was fed into a dip basin and the pickup was controlled
by the smooth metering rod on each side of the fabric. Oven
temperatures were: 121.degree. C., 177.degree. C., and 260.degree.
C. Coating speed was 5 feet/min. The fabric was then dipcoated with
a 1.45 specific gravity aqueous dispersion of PTFE to which was
added 5% based on PTFE solids of 3-aminopropyltriethoxysilane. The
PTFE aqueous dispersion was coated using two sets of smooth bars to
apply the dispersion. Oven temperatures were as follows:
121.degree. C., 204.degree. C., and 391.degree. C. Coating speed
was 3 feet/min. The fabric was then coated repeatedly using a
multiple pass process with a ceramic filled aqueous PTFE
dispersion. The ceramic dispersion contained titanium dioxide,
PTFE, 3-aminopropyltriethoxysilane, a ceramic dispersing agent, a
non-ionic surfactant, a strong organic acid, and a perfluorinated
poly(tetrafluoroethylene-alkylvinylether) copolymer. Coating speed
varied from 4-8 feet/min. Oven temperatures were 93.degree. C.,
204.degree. C., and 399-407.degree. C. Coating speeds were from 3-8
feet/min. The 7628-508 style fiberglass was coated to a final
weight of 0.95 lbs/yd.sup.2 to yield a smooth sheet. The sheet was
obtained on a roll and was treated with a sodium ammonia etching
compound to activate the surfaces.
Example 2
Preparation of an Epoxy Coated, Fluoropolymer Impregnated Woven
Glass Fabric
[0079] An epoxy formulation was prepared by blending a catalyst
composition with an epoxy resin formulation. The catalyst
composition was prepared by mixing 1.98 kg of methylether ketone
solvent, 0.184 kg of a non-ionic surfactant (Pluronic L92 available
from the BASF Corporation), and 0.0369 kg of manganese
2-ethylhexanoate (available from OMG Americas). The epoxy
formulation was prepared by blending the following: 40.147 kg of
Dow 538-A80 (a glycidyl ether of brominated bisphenol A available
from the Dow Chemical Company), 38.87 kg of BT2110 (a
bismaleimide/bisphenol A dicyanate oligomer available from the
Mitsubishi Gas Chemical), 13.9337 kg of DER560 (a brominated epoxy
resin available from the Dow Chemical Company), 5.379 kg of Shell
Epon 55-BH-30 (a bisphenol A based epoxy available from the Shell
Oil Company), 12.16 kg of dimethylformamide solvent, 9.0 kg of
methylether ketone, 9.0 kg of propylene glycol methyl ether
acetate, and 5.7 kg of Phenoxy PKHS-40 (a poly(hydroxyether)
available from Inchem Corp). Prior to use, 136 kg of the epoxy
formulation was mixed with 2.20 kg of the catalyst solution.
[0080] The thermosetting resin solution was coated onto both sides
of the fluoropolymer coated 7628 fabric using a Litztler dual
reverse roll coater. The thermoset was applied using a 13 mil gap
between the two reverse rolls. The Litzler had two oven sections.
Zone 1 was 90.degree. C. while Zone 2 was 165.degree. C. The
Litzler is a vertical treater with one 7.5 meter length zone
extending vertically connected to another 7.5 meter zone that
returns to the base of the treater. The therinoset was coated at
2.5 meters/minutes resulting in a 3 minute dwell time in each of
the high and low temperature ovens. The dried prepreg had a final
10 mil thickness, 9 mil from the base fluoropolymer coated fabric,
and 0.5 mil per side of the thermosetting resin.
[0081] 3 plies of the prepreg were stacked up and pressed at
176.6.degree. C. for 10 minutes at 300 psi. The prepreg was weighed
before and after pressing. The resin that squeezed out of the press
was collected and weighed. 3.5% resin flowout was obtained.
[0082] Two plies of the material were sandwiched between 2 pieces
of 1 oz. zinc free foil and pressed between skived PTFE sheets that
were used for release purposes. The press conditions were 176.6C
for 2 hours at 300 psi. The copper cladding was etched off and the
samples were dried. The following electrical properties were
obtained: dielectric constant was 3.50 (IPC-TM 650 2.5.5,1 MHz),
dielectric loss of 0.0047 (IPC-TM 650 2.5.5, 1 MHz), dielectric
constant at 10 GHz of 3.41 (IPC TM 650 2.5.5.5), dielectric loss at
10 GHz of 0.0055 (IPC TM 650 2.5.5.5). Peel strength was 13.6
lbs/linear inch (IPC-TM 650 2.4.8). Moisture absorption was 0.28%
after 24 hour immersion. Moisture absorption after 1 hour of
pressure cooker exposure was 0.84%. This example demonstrates that
very attractive electrical properties can be obtained even in the
absence of a ceramic filler that might be added to the
thermosetting resin to offset the less desirable loss tangent
properties of the thermosetting resin itself.
Example 3
Preparation of a Hybrid 4 Layer Multilayer Circuit Board Using the
Bond Ply from Example 2.
[0083] The inner layers of (1) a 21 mil PTFE coated fiberglass
laminate clad on both sides with 0.5 oz copper and (2) a 48 mil
FR-4 epoxy laminate clad on both sides with 0.5 oz copper were
patterned and etched according to the following (subtractive
process) standard printed circuit board procedures: punch, holes
drilled, hole deburr, scrub & coat inner layers with
photoimaging material, expose inner layers, develop inner layers,
and strip inner layers. The inner layers of the FR-4 and PTFE
fiberglass were then treated with metal oxide, baked, and laminated
together using the multilayer bond ply prepreg from Example 2.
Press conditions were 176.degree. C. for 2 hours at 300 psi. The 4
layer multilayer was then treated with the following standard outer
layer print and etch processes: drilled, deburred, baked, treated
for etchback, sodium napthalene etch, electroless plated,
photomaterial lamination, expose outer layers, develop outer
layers, pattern plate copper and then tin, resist strip, and etch
outer layers. A second imaging step was done as follows: bake dry
film and apply, coat outer layers, expose outer layers, develop
outer layers, tin/lead strip, strip resist, bake before reflow,
solder reflow, coat liquid photoiamageable solder mask, expose
solder mask, develop solder mask, and UV cure solder mask.
[0084] Examples of the gap filling ability of the multilayer
prepreg bond ply from Example 2 can be found in FIGS. 2 and 3.
FIGS. 2 and 3 confirm the desired gap filling properties of the
bond ply. Microsectioning of the hybrid board did not reveal the
presence of any voids between the copper circuitry. This example
demonstrates that multilayer hybrid boards can be prepared at
conventional thermosetting resin conditions using a
fluoropolymer-fiberglass-epoxy resin composite B-staged bond ply as
the adhesive layer.
Example 4
Preparation of a Hybrid 4 Layer Multilayer Circuit Board Using a
106 Style Fiberglass Composite Bond Ply
[0085] Examples 1 and 2 were repeated with the exception that the
base PTFE composite was a PTFE coated 106 style woven fiberglass
that was coated in a multi pass process to a 60% PTFE content based
on the total composite (1.45 mil thickness). The material was then
etched using a sodium ammonia etching process. Thermosetting resin
was applied to both sides using the previously described
procedures. The gap between the reverse rolls was 10 mil that
yielded a product having a thickness of approximately 2.5 mils. A 4
layer multilayer was made using the procedure described in Example
3. The gap filling properties of the bond ply are shown in FIG. 3.
FIG. 3 confirms the desired gap filling of the bond ply. This
example also demonstrates that the invention disclosed herein is
applicable to very thin substrates.
Example 5
Preparation of a Fluoropolymer Impregnated Woven Fiberglass
Composite using a Talc filled Epoxy as the Bonding Layer
[0086] Example 2 was repeated except that 86.5 kg of talc (Benwood
talc available from Zeinex Fabi Benwood, LLC) is dispersed into the
thermosetting resin formulation to reduce the impact of the
thermosetting resin on the dielectric loss properties of the final
laminate. The talc filled thermosetting resin solution is coated
onto the 7628 style fiberglass using the previously described
coating method. The talc filled bond ply is used as described in
Example 3 to make a 4 layer multilayer. The example demonstrates
that ceramics can be incorporated into the thermosetting resin
layer to reduce cost; x, y, and z coefficients of thermal
expansion; and dielectric loss properties.
Example 6
Preparation of a Fluoropolymer Coated Non-Woven Glass Fabric
[0087] 1.5 mil non-woven polyaramide fabric (Thermount, available
from Dupont) made from pulped and/or staple Kevlar or Nomex fibers
is coated under low tension according to Example 1 to a 70% PTFE
resin content using a PTFE aqueous dispersion. The surface of the
composite is sodium naphthalene etched and coated with
thermosetting resin according to the process described in Example
2. A 4 layer multilayer is prepared using the procedure outlined in
Example 3. This examples demonstrates that a bond ply can be
prepared from a non-woven substrate that is suitable for high
density interconnect packages such as: multichip modules, ball grid
array packages, direct chip attach, and ultra fine lines and
spaces. This example further demonstrates that a substrate can be
produced that is well suited for laser drilling.
Example 7
Preparation of a Fluoropolymer Impregnated Non-Woven Glass Fabric
Surface Coated with a kaolin filed Thermosetting Resin
[0088] Example 6 is repeated with the exception that the
thermosetting resin contains a ceramic filler, kaolin. This example
demonstrates that a ceramic filler can be used to reduce cost; x,
y, and z coefficients of thermal expansion; and dielectric loss
properties in high layer count multilayers and packages
incorporating fine lines and spaces (multichip modules, ball grid
array packages, and direct chip attach).
Example 8
Preparation of a Bond Ply using a Fiberglass Composite Prepared
using a Hot Roll Laminator
[0089] Example 4 is repeated with the exception that the 106 style
fiberglass is not fully impregnated with a PTFE dispersion. Two
separate 1.0 mil skived PTFE films are pressed onto the two sides
of a lightly PTFE coated 106 style fiberglass using a hot roll
laminator operating at 375C. A thermosetting resin is applied
according to the previously described procedures and a 4 layer
multilayer PWB is prepared. This example demonstrates that a more
cost effective method can be used to apply the PTFE component to
the reinforcing component of the bond ply.
Example 9
Preparation of a Ceramic Filled Bond Ply using a Fiberglass
Composite Prepared from a Hot Roll Laminator
[0090] Example 8 is repeated with the exception that the skived
PTFE contains a ceramic filler. This example demonstrates that a
ceramic filled PTFE film can be laminated onto a lightly coated
fiberglass to increase key properties in a cost efficient
manner.
Example 10
Preparation of a Bond Ply using a Non-woven Reinforcement Prepared
using a Hot Roll Laminator
[0091] Example 8 is repeated with the exception that a 1.5 mil film
of poly(perfluorinated alkylvinylether-tetrafluoroethylene
copolymer) is hot roll laminated into a non-woven polyaramide
fabric creating a flat substrate. The bond ply is manufactured by
sodium napthalene etching the sheet followed by application of the
thermosetting resin as previously described. The 4 layer multilayer
is prepared using the previously described procedures. This example
demonstrates that a non-woven reinforced fluoropolymer composite
can be prepared in a cost efficient manner by impregnating a
nonwoven fabric with a melt flowable fluorinated polymeric film
using a hot roll laminator.
Example 11
Preparation of a Bond Ply using a Non-woven Reinforcement
[0092] PBO staple and pulped fiber is added to a PTFE dispersion.
The filled dispersion is then dipcoated in a multipass process onto
a 5 mil polyimide carrier film using the temperature conditions
outlined in Example 1. The PTFE coated non-woven substrate is then
calendered to form a 2 mil flat sheet at 371.degree. C. using a
flame-heated hollow cylinder. The sheet is then consolidated from
the polyimide carrier film. The sheet is then surface treated using
a sodium naphthalene etching solution. The flat sheet is then
surface coated with the thermosetting resin composition from
Example 2. A 4 layer multilayer is built according to the procedure
of Example 3. This example demonstrates that a laser drillable
substrate containing PTFE can be prepared from a fiber filled PTFE
dispersion yielding a thin low loss composite having controlled x,
y, and z coefficient of thermal expansion values.
Example 12
Preparation of a Fluoropolymer Coated Non-Woven Fiber Glass
Fabric
[0093] Non-woven fiberglass, 0.007 "and 0.015" thick respectively,
(available from Lydal Manning as Manninglas.RTM. styles 1201 and
1200 respectively) was coated under low tension according to
Example 1 to an 83 to 86% PTFE resin (available from 1. E. DuPont)
content using a 1.400 specific gravity PTFE aqueous dispersion at 1
to 8 fpm using a smooth metering rod. A subsequent topcoat of PFA
dispersion was coated onto the substrate using a 1.400 specific
gravity at 3-5 fpm to a finished resin content of 88-91%. The
surface of the composite is sodium naphthalene etched and coated
with thermosetting resin according to the process described in
Example 2. A 4 layer multilayer is prepared using the procedure
outlined in Example 3. These examples demonstrate that a bond ply
can be prepared from a non-woven substrate. That substrate has
ample "good" electrical resin added in much fewer manufacturing
steps. The non woven reinforcement is not a fiberglass matrix,
therefore no glass fiber window "voids" from the weaving process
exist which makes a woven product less homogeneous. This provides
for a more uniform product that is suitable for high density
interconnect packages such as: multichip modules, ball grid array
packages, direct chip attach, and ultra fine lines and spaces. This
example further demonstrates that a substrate can be produced that
is well suited for laser drilling.
Example 13
Preparation of a Fluoropolymer Impregnated Non-Woven Fiber Glass
Composite using a Talc filled Epoxy as the Bonding Layer
[0094] Example 12 is repeated except that the thermosetting resin
of Example 5 is used to coat the non-woven substrate. The talc
filled bond ply is used as described in Example 3 to make a 4 layer
multilayer. The example demonstrates that ceramics can be
incorporated into the thermosetting resin layer to reduce cost; x,
y, and z coefficients of thermal expansion; and dielectric loss
properties. This example further demonstrates that a substrate can
be produced that is well suited for laser drilling.
Example 14
Preparation of a Ceramic Filled Fluoropolymer Coated Non-Woven
Fiber Glass Fabric
[0095] Non-woven fiberglass, 0.007 "and 0.015" thick respectively,
(available from Lydal Manning as Manninglas styles 1201 and 1200
respectively) were coated under low tension according to Example 1.
A 25 wt % loading of TiO.sub.2 [ Available from SCM.
TIONA.sub..RTM. RCS-9 rutile TiO2 slurry and 25 wt % of SiO.sub.2 [
available from ITC-SiO.sub.2 with 0.5% A-187 fluorosurfactant as a
surface treatment] was used by weight in a PFA [perfluorinated
alkylvinylether] (available from I. E. DuPont) aqueous dispersion.
The ceramic filled dispersion was broken down with water to 1.400
specific gravity and coated from 0.5-2 fpm to achieve a total resin
content of 80 to 90%. No top coating and the abundance of ceramic
obviate the need for the etching process before the application of
the thermosetting bonding layer. A 4 layer multilayer is prepared
using the procedure outlined in Example 3. This example
demonstrates that a bond ply can be prepared from a non-woven
substrate that is suitable for high density interconnect packages
such as: muitichip modules, ball grid array packages, direct chip
attach, and ultra fine lines and spaces. This example further
demonstrates that a substrate can be produced that is well suited
for laser drilling. This example further demonstrates that with
highly filled ceramic composites the chemical etching process is
not always required.
Example 15
Preparation of a Ceramic Filled Fluoropolymer Impregnated Non-Woven
Fiberglass Composite using a Talc filled Epoxy as the Bonding
Layer
[0096] Example 14 is repeated except that the thermosetting resin
of Example 5 is used to coat the non-woven base ply. The talc
filled bond ply is used as described in Example 3 to make a 4 layer
multilayer. The example demonstrates that ceramics can be
incorporated into the thermosetting resin layer to reduce cost; x,
y, and z coefficients of thermal expansion; and dielectric loss
properties. This example further demonstrates that a substrate can
be produced that is well suited for laser drilling.
Example 16
Preparation of a Laminate Consisting of alternating Layers of Low
Loss Substrate and Bonding Plies (Low Loss Substrate with Adhesive
Layer).
[0097] 1080 fiberglass was coated as in Example 1 with first a
based coat of pure PTFE aqueous dispersion, followed by a series of
PTFE dispersions containing ceramic additives. The surface of the
PTFE-fiberglass was treated as described in prior sections. The
fiberglass was coated to a weight of 0.5 lbs/yd.sup.2. A similar
thermosetting resin as described in Example 2 was coated onto both
sides of the fabric yielding a bonding ply as described in FIG. 1.
The thermosetting resin contained fused silica. The bond ply was
coated with the thermosetting resin using two opposing wrapped wire
metering rods to a weight of 0.6 lbs/yd.sup.2. This bond ply was
combined with a PTFE coated fiberglass that contained no
thermosetting resin. 2116 style fiberglass having a base weight of
0.191 lbs/yd.sup.2 was sequentially coated to a weight of 0.6
lbs/yd.sup.2 with PTFE. This PTFE-fiberglass substrate was surface
treated using the techniques described above. The following
laminate was constructed: copper/thermoset treated
PTFE-fiberglass/surface treated PTFE-fiberglass/thermoset treated
PTFE-fiberglass/surface treated fiberglass/thermoset treated
PTFE-fiberglass/copper. The laminate was pressed at 900 psi at
392.degree. F. using a 10.degree. F. rate of rise and a 2-hour hold
cycle at temperature. The product had a dielectric constant of 2.94
at 10 GHz and a dissipation factor of 0.0034 at 10 GHz. A separate
laminate was built using 5 layers of the thermoset treated
PTFE-fiberglass of this example. This laminate had a dielectric
constant of 3.28 and a dissipation factor of 0.004 at 10 GHz, both
of which were flat over frequency. This example demonstrates that
when surface treated PTFE-fiberglass is interleafed with thermoset
treated PTFE-fiberglass the following can be accomplished (1) the
dielectric constant can be further reduced (2) the dissipation
factor can be further reduced and (3) the cost can be reduced
because there is a reduction in the amount of PTFE-fiberglass that
needs to be treated with an adhesive layer.
Example 17
Preparation of a Laminate Consisting of alternating Layers of Low
Loss Substrate and Bonding Plies (Low Loss Substrate with Adhesive
Layer).
[0098] The experiment of Example 16 was repeated with the exception
that a fluoropolymer film, PTFE, was interleafed with layers of
PTFE-fiberglass-thermosetting resin. The construction was a
follows: copper cladding, thermoset treated PTFE-fiberglass,
surface treated 20 mil layer of skived film, thermoset treated
PTFE-fiberglass, copper cladding. This example demonstrates that
pure fluoropolymer can be inserted into the laminate to reduce
dielectric constant, dissipation factor, and cost.
Example 18
Preparation of a Laminate Consisting of alternating Layers of
Reinforcement and Bonding Plies (Low Loss Substrate with Adhesive
Layer).
[0099] The experiment of Example 16 was repeated with the exception
that a layer of 7628 based FR-4 prepreg, 0.008", was intermixed
with the 0.006" PTFE-fiberglass-thermosetting adhesive. The
construction was a follows: copper cladding, thermoset treated
PTFE-fiberglass, FR-4 prepreg, thermoset treated PTFE-fiberglass,
copper cladding. This example demonstrates that the cost of the
laminate can be significantly reduced by interleafing plies of FR-4
or some other reinforcement or thermoset impregnated
reinforcement.
Example 19
Preparation of a Laminate containing an Injection Moldable Grade of
Fluoropolymer in the First Impregnation of the Reinforcement.
[0100] This example was carried out in a similar fashion to Example
1 with the exceptions 1080 fiberglass was used that the first pass
coating of the fluoropolymer comprised a fluoropolymer dispersion
of MFA or PFA, copolymers of tetrafluoroethylene and either of
perfluorinated propyl vinyl ether or perfluorinated methyl
vinylether, and 3-aminopropyltriethoxysilane. The
3-aminopropyltriethoxysilane zzwas present in a 5 phr based on the
solids weights of the silane and the fluoropolymer. This example
demonstrates that an injection moldable grade of fluoropolymer can
be used in the first fluoropolymer dispersion pass.
Example 20
Preparation of a Laminate containing a PTFE particle that has been
Crosslinked
[0101] This example was carried out in a similar fashion to Example
1 with the exception that either solid particles of crosslinked
PTFE or similar particles dispersed in an aqueous emulsion were
added to the fluoropolymer dispersions. This example demonstrates
that a reduced coefficient of thermal expansion can be obtained by
adding crosslinked fluoropolymer particles to non crosslinked
fluoropolymers. Example 21
Preparation of a Laminate containing a Crosslinked PTFE particle
that has been Incorporated into the Thermosetting Adhesive
Component
[0102] This example was carried out in a similar fashion to Example
5 with the exception that either solid particles of crosslinked
PTFE or similar particles dispersed in some medium emulsion were
added to the thermosetting adhesive. This example demonstrates that
a reduced dissipation factor can be obtained by adding crosslinked
fluoropolymer particles to the thermosetting adhesive.
Example 22
Preparation of a Laminate containing a Plurality of Ceramics in the
PTFE-fiberglass Layer and a Ceramic in the Adhesive Layer.
[0103] Examples 1, 2, and 3 are repeated with the exceptions that
the 2.sup.nd pass and subsequent coating of the fluoropolymer onto
the woven fiberglass substrate contained 50 wt % of a ceramic
dielectric modifier that comprised amorphous fused silica and
titanium dioxide. 1080 style woven fiberglass is coated to 0.14
lbs/yd.sup.2 with a dispersion of silane and
polytetrafluoroethylene. The substrate is then coated to a weight
of 0.5 lbs/yd.sup.2 in multiple passes using a ceramic
fluoropolymer dispersion comprising PTFE, amorphous fused silica,
and fiberglass. The surface of the substrate is then treated using
previously described techniques. The substrate was then coated with
a similar thermosetting adhesive composition described in Example 2
with the exception that the adhesive contained >50 wt % of
amorphous fused silica. The PTFE-ceramic-fiberglass substrate was
coated to a weight of 0.6 lbs/yd.sup.2. A laminate was produced
from this substrate by pressing at 450 psi, 392.degree. F., for two
hours after a 10.degree. F/minute ramp rate. This laminate yielded
a dissipation factor of 0.004 and a dielectric constant of 3.27
Example 23
Preparation of a Bondply having varying Levels of Thermosetting
Resin
[0104] 1080 fiberglass was coated in a similar fashion to Examples
1 and 22. The bare weight of 0.088 lbs/yd.sup.2 was coated to 0.13
lbs/yd.sup.2 with polytetrafluoroethylene dispersion. The substrate
was then coated to 0.3 lbs/yd.sup.2 with multiple coating passes of
a polytetrafluoroethylene dispersion containing 17 wt % (solids on
solids) of titanium dioxide. The substrate was then surface treated
using the previously described methods. A fused silica modified
thermosetting resin similar to Example 2 was applied to the
surfaces of the substrate using 0.024" wrapped wire rods, 9.3
ft/min. This thermosetting resin formulation was modified such that
chain extended epoxy resins were drastically reduced or completely
eliminated and replaced with di glycidyl ether analogues. The
weight increased from 1.69 g/16 in.sup.2 to 2.25 g/16 in.sup.2
yielding a thermoset coated PTFE-fiberglass substrate having a
thermoset content of 25%. The same PTFE-fiberglass substrate was
coated to a 2.6 g/16 in.sup.2 weight using 0.032" wrapped wire rods
yielding a thermosetting resin content of 35 wt %. Laminates were
prepared from the respective bondplies by stacking a multitude of
layers together and laminating at 392.degree. F. for one hour using
a 10.degree. F. rate of temperature rise. The dependence of the
dissipation factor and dielectric constant on frequency can be seen
in Figure 10. This example demonstrates that either a laminate or
bondply can be designed with the desired dissipation factor less
than 0.005. This example also demonstrates that that if the
thermosetting adhesive layer is allowed to increase beyond the 35
wt% content, it is more difficult to maintain a dissipation factor
less than 0.005. This example also demonstrates the impact of the
fused silica modifier to the thermosetting layer. Fused silica has
a dissipation factor of 0.0045 versus 0.017 for typical epoxy based
thermosets. If the same laminate did not comprise fused silica in
the thermosetting adhesive layer, a dissipation factor less than
0.005 could not be achieved.
Example 24
Preparation of a High Flowing Conformal Low Loss Substrate
Bonding
[0105] A 3 mil PTFE-fiberglass substrate was prepared using
previously described methods by coating standard 1080 fiberglass
with first a PTFE dispersion and subsequently ceramic filled
aqueous dispersions to a weight of 0.28 lbs/yd.sup.2. This was
surface treated using previously described techniques. A high
flowing thermosetting resin was applied using previously described
methods. The high flowing thermosetting resin solution consisted of
204 lbs of amorphous fused silica, 85.4 lbs BT2110 resin, 53.3 lbs
Epon Resin 826 (a non advanced diglycidylether derivative of
bisphenol A), 13.4 lbs DER560 (a brominated diglycidylether
derivative of bisphenol A, a minor portion of Dow538-A80 brominated
resin (15.79 lbs), 50 lbs of a solvent (propylene
glycolmethyletheracetate), catalyst (manganese octanoate), and a UV
active agent (Uvitex OB). The solution viscosity before treating
was adjusted to 1.280 using propylene glycolmethyletheracetate. The
PTFE-fiberglass substrate was coated taaao 0.36 lbs/yd.sup.2 with
the thermosetting resin using a two zone treater with temperatures
of 240.degree. F. and 280.degree. F., at a speed of 9.3 ft/min. The
thermosetting resin was applied with wrapped wire wound rods. This
substrate was then used as a bonding prepreg to make multilayers
shown in FIG. 11. The FR-4 inner layers were based on 1 oz copper.
Lamination was conducted at 392.degree. F., using a 10.degree.
F/minute rate of heat rise. FIG. 11 demonstrates that the
PTFE-fiberglass layer when combined with a very high flowing
thermosetting resin system, the PTFE-fiberglass layer can be
reformed during lamination and can conform to the outlines of
copper circuitry thus reducing the minimum amount of thermosetting
resin necessary to fill difficult circuitry.
[0106] It is unexpected that the PTFE-fiberglass layer will
participate in the gap filling of the circuitry in addition to the
high flowing thermosetting resin system.
* * * * *